1. Field of the Invention.
The present invention relates to a method of coating for a nitride ceramic member such as silicon nitride, aluminum nitride or the like.
2. Description of the Related Art.
Since ceramic materials have good properties such as mechanical and chemical properties, these materials have been popular in a wide application range of structural materials and functional materials. In many applications, components consist of only ceramic materials. However, in order to utilize ceramic materials in a still wider application range, metals must be bonded to ceramic members. A conventional method of effectively bonding a ceramic member and a metal is a method of metallization for a ceramic member as follows.
First, a powder containing Mo or W as a major constituent is applied to a surface of a ceramic base, heated at 1,400.degree. to 1,700.degree. C. in a reducing atmosphere, and reacted with Mo or W to achieve metallization.
Second, an Au or Pt film is placed on a surface of a ceramic base and heated while the film is compressed, thereby achieving metallization of the ceramic base.
Although the first method is suitable when Al.sub.2 O.sub.3 is used as a base material, an adhesion force with the metal is weak in a nitride ceramic material (e.g., aluminum nitride (AlN) or silicon nitride (Si.sub.3 N.sub.4) which has recently received a great deal of attention as a high-performance ceramic material.
According to the second method, an expensive noble metal is used to cause an economical disadvantage. In addition, a high pressure is required to improve adhesion strength between the base and the metal. Therefore, the second method is not suitable for metallization of a member which is not to be deformed.
A direct metallization method for a member consisting of a nitride ceramic such as Si.sub.3 N.sub.4 is proposed (U.S. Pat. No. 4,685,607). This method utilizes slight decomposition of Si.sub.3 N.sub.4 at a high temperature. For example, a metal powder which is easily alloyed with Si is sprayed or coated on a surface of an Si.sub.3 N.sub.4 member and is heated at a temperature of 1,150.degree. C. or higher and a pressure of 5.times.10.sup.-4 Torr or less to form a metallized layer. More specifically, on the surface of the Si.sub.3 N.sub.4 member, Si produced by decomposition of Si.sub.3 N.sub.4 is diffusion-reacted with the metal coated on the surface of the Si.sub.3 N.sub.4 member, and a liquid phase spreads due to a decrease in melting point by the diffusion reaction. As a result, a metallized layer consisting of an alloy of Si and the sprayed or coated metal is formed.
According to this method, however, it is difficult to control the grain size and thickness of the metal powder sprayed or coated on the surface of the Si.sub.3 N.sub.4 member. An excess amount of the metal powder is often present on the surface of the Si.sub.3 N.sub.4 member. If the excess amount of the metal powder is present on the surface of the Si.sub.3 N.sub.4 member, the thickness of the metallized layer is increased. A large thermal stress is generated during cooling of the metallized layer from the metallizing temperature to room temperature, and the Si.sub.3 N.sub.4 member may be cracked.
AlN as another typical nitride ceramic material has received a great deal of attention as a semiconductor device substrate due to a high thermal conductivity. In this case, bonding of AlN with a conductive material is indispensable. A metal which is easily alloyed with Al is sprayed on the surface of the AlN member and is heated at a temperature of 1,150.degree. C. or higher and a pressure of 5.times.10.sup.-4 Torr or less in the same manner as in Si.sub.3 N.sub.4.
When the above method is employed, the following disadvantage is caused. Since an amount of Al produced by decomposition on the surface of the AlN member is very small, a diffusion reaction between Al and the sprayed or coated metal does not easily occur. It is therefore difficult to form a metallized layer having a sufficient thickness. An attempt has been made to decompose AlN in a sufficient amount at a temperature of 1,500.degree. C. or higher. When the decomposition is performed at such a high temperature, the sprayed or coated metal is converted into a liquid phase to cover the surface of the AlN member. Production of Al by AlN decomposition is interfered to disable formation of the metallized layer.
TiN as a conductive ceramic material has recently received a great deal of attention as a conductive material formed on an AlN substrate. Since TiN is a ceramic material, it has a thermal expansion coefficient equivalent to that of AlN unlike a metal used for metallization. TiN has a small thermal stress acting on AlN, resulting in convenience. Chemical vapor deposition (CVD) or physical vapor deposition (PVD) can be used as a method of coating TIN on AlN. However, TiN is only mechanically adhered to the surface of AlN. Therefore, a sufficient adhesion strength cannot be obtained, and the obtained metallized layer does not have a satisfactory function.